1. Field of the Related Art
The present invention relates to a writing or overwriting magnetic head for magneto-optical recording.
2. Description of the Related Art
Magneto-optical recording should be subject to considerable growth in the next few years. Positioned between magnetic recording and optical memories, the magneto-optical process makes it possible to write and read at random binary data on a support. As a result of the high information densities stored on a removable or exchangeable disk, this procedure makes it possible to compete with hard computer disks and in particular floppy disks.
Although the performance characteristics as regards access or retrieval time of a magneto-optical device remain inferior to those of a hard magnetic disk (20 ms compared with approximately 8 ms for a hard disk), the advantages of the removability of the disk, the information storage stability (min 15 years and even 40 years as a function of the magneto-optical support type as compared with 2 to 3 years for a magnetic floppy disk) and the great information density (ISO standard 128 Mo for diameter 88 mm disks, or a density of approximately 16 kTPI and 13 kBPI, new standard in the course of definition roughly double the capacity of the ISO standard, more than 100 Mo for the Mini-Disc of 64 mm of SONY).
The fields of application of the invention are computer bulk memories (microprocessing, work stations and large systems), the storage of computer data on removable floppy disks (software, data bases, information filing) and general public audio/video applications (SONY Mini-Disc or future rerecordable video disks).
The magneto-optical recording method is derived from that of so-called compact optical disk readers. As in the case of the latter, it requires a laser diode, an optical head, a disk drive system and an electronics mainly of a logic nature. However, the optical head is more complex to produce, because the reading of the solely optical information calls on the detection of the rotation of the polarization plane of the reflected light (KERR effect) or transmitted light (FARADAY effect).
Most existing magneto-optical recorders operate in two stages for writing, namely an erase phase, during which a magnetic field of given direction is applied, followed by a write phase during which the magnetic field is reversed. The erase phase is necessary when it is not possible to sufficiently rapidly reverse the direction of the magnetic field. The direct overwrite process is expected to come into general use in the near future, because it avoids the prior erase phase and thus minimizes the writing time (total recording time divided by two).
At present there are two direct overwrite processes, namely laser beam modulation or LBM and magnetic field modulation or MFM. The first process requires specific disks (having several magnetic coatings), as well as a variation of the laser power between two levels, but it avoids having to vary the magnetic field. The second process, to which the present invention applies, makes it possible to write on the entire magneto-optical disk whilst very rapidly modifying the magnetic field (in less than a fraction of a microsecond, which corresponds to frequencies of several megahertz).
In order to be effective, the head ensuring the variation or modulation of the magnetic field must satisfy various criteria:
creation of an adequate magnetic field (approximately 20 mT) in the magneto-optical coating of the disk at the laser focusing point,
limited inductance (approximately 1 .mu.H) in order to permit a sufficiently rapid variation (square wave pulses of a fraction of a microsecond, or a fraction of a tenth of a microsecond, which is equivalent to frequencies of a few MHz to 50 MHz),
compatibility with a removable disk, which can therefore carry dust,
limited mass in order to ensure a minimum access time,
good heat dissipation.
The magnetic heads used at present in magneto-optical recorders for the modulation of the magnetic field (particularly for the SONY Mini-Disc) have a relatively large size (several mm). Therefore these heads suffer from the disadvantage of having a high inductance and are therefore limited in the modulation frequency and therefore in the writing rate. Their considerable size is also accompanied by a considerable weight leading to a not very good access time.
EP-A-492 888 describes a thin film magnetic head comprising a magnetic material substrate in which a recess is formed in order to receive a conductor coil. This coil is flush with the level of the lateral edges and the central pole of the magnetic substrate.
The attached FIG. 1 shows a magnetic head according to this prior art. The magnetic head substrate is 2. It has a recess between an outer edge 3 and a central pole 4, the latter being surrounded by the coil 5. The assembly is placed upstream of a support 6 coated with a recording coating 7. The magnetic field lines are 8. They intersect the recording coating 7 in a relatively wide area, but it is essentially the area facing the pole 4 which is subject to writing, the field rapidly becoming very weak on moving away from it. Moreover, the writing or overwriting area is more specifically defined by a laser beam 9, which strikes a very narrow portion of the coating 7, where it is wished to write a data element.
This type of head suffers from disadvantages. The dissipation of the heat given off by the coil is not very adequate. In the case of a flight or movement accident, when the head collapses or crashes on the coating 7, the winding 5 could be seriously damaged. Moreover, such a head has a significant weight due to the nature of the substrate (made from a ferrite such as nickel-zinc ferrite), which reduces the access time. Finally, the production process of such a head with the machining of its substrate is not very appropriate for mass production, so that the head price is high.
The aim of the present invention is to obviate these disadvantages.